Agricultural operations such as harvesting, sowing, spraying and seeding can benefit greatly by having the relevant vehicle such as a harvester, tractor or sprayer following a precise path in order to minimise overlap and/or to allow precise placement with respect to previous operations. These operations are often conducted over terrain which may be undulating or have locally varying characteristics which can affect the operation of the vehicle. As such, vehicle guidance systems have been adopted to guide these vehicles especially for large scale farming where extensive tracts of land are involved or where very precise placement of the implement or machinery can be of value. These systems ensure that the land is traversed by the vehicle in the most efficient manner following an optimised predetermined path as manual operation of an agricultural vehicle may result in gaps or the non optimal use of fuel or agricultural materials.
Typically, these vehicle guidance systems will involve the reception of a position indication signal which allows a receiver located on the vehicle to determine its position. Commonly, the position indication signal is taken from a Global Navigation Satellite System (GNSS) satellite based positioning system such as the American Global Positioning System (GPS) and/or the Russian GLONASS System and/or the European Galileo system signal or the more accurate differential satellite signal. This allows a satellite receiver located on the vehicle to determine the vehicle's absolute position. In more precise applications, the guidance system may also include gyroscopes, a compass and accelerometers located on the vehicle. This also allows the guidance system to infer more accurately the position of the towed implement as opposed to the position of the GNSS antenna which is typically mounted on the vehicle roof and so subject to extra movements of the vehicle as it passes over the terrain. It also allows guidance to continue when there is a temporary interruption to the satellite signal.
Alternatively, the position indication signal may be based on a network of local radio transmitters so that a vehicle can determine its relative position with respect to a number of fixed reference points by triangulation or other means. In another alternative, the position indication signal may be a control signal indicating in which direction the vehicle should be moving. Based on the position indication signal, a steering control signal is then generated which controls the vehicle steering system to follow the predetermined path.
In one example, the steering control signal is linked into an additional hydraulic valve that has been incorporated into the steering system of the vehicle which directly drives the mechanical steering assembly of the vehicle. In many instances, systems of this type will involve substantial retrofitting and modification of the hydraulic steering system which, as well as involving a significant added expense to the vehicle, will also further complicate the hydraulic steering system. This can lead to associated maintenance and reliability issues due to the added complexity of the additional fitted control system and its associated components. In addition, systems of this type offer no portability between different types of vehicles which may require a vehicle guidance system as modifications made will be specific to each type of vehicle.
Other examples of vehicle guidance systems are directed more closely to the manual operable portion of the steering assembly by either driving the steering shaft or indirectly actuating the steering wheel by a remote drive which is then coupled to the steering wheel of the vehicle. Examples of these remotely coupled systems include a belt or chain drive arrangement incorporating a separate standalone motor which drives a pulley or chain connected to the steering wheel. Another example includes a ring gear arrangement where a separate standalone motor drives a ring gear mounted to the steering wheel. In another example, the coupling between the separate motor and the steering wheel is by frictional engagement with the periphery of the steering wheel. In yet another example, a worm gear arrangement is employed to couple the standalone motor to the steering wheel.
All of these examples which attempt to actuate the steering wheel involve some modification to the standard manually operable steering assembly of a vehicle. Accordingly, these systems can be expensive and time consuming to both install and remove. Another important disadvantage of these systems is that they are inherently limited in how rapidly they are able to turn the steering assembly. This is turn limits the speed at which steering corrections to the direction of the vehicle can be achieved resulting in overall reduced accuracy and impacting on the maximum vehicle speed that a vehicle can be operated. Also, as space is often at a premium in the cabin of a vehicle, these systems have the added disadvantage of the motor and coupling arrangements occupying extra room within the vehicle which can present a safety hazard. Another related disadvantage is that the motor and coupling arrangements result in extra environmental noise in the cabin of the vehicle.
There is therefore a need for a vehicle guidance system that is capable of being conveniently fitted to the steering system of a vehicle in a simple and more cost effective manner. There is similarly, a need for a vehicle guidance system capable of supplying sufficient torque to the steering mechanism to provide accurate control over a range of vehicle speeds. There is also a need for a vehicle guidance system that does not otherwise impact upon the operator of the vehicle.